Personal active noise cancellation method and device having invariant impulse response

ABSTRACT

A selected electrical signal is first applied to the input of a source (30) to determine at least the impulse response (H), over a predetermined time, between the source output and the input of sensors (20). The values of the active cancellation control signal are determined in real time according to a predetermined law established in accordance with the previously determined impulse response (H), whereby the energy of the output signal from the sensors is minimised. The acoustic structure (2) is arranged so that the space (534) containing the sensors (20) and the source (30) is invariant at least during the determination of the active cancellation control signal and in the presence of a person, whereby the impulse response (H) between the output of the source (30) and the input of the sensors (20) may remain invariant.

The present invention relates to active acoustic attenuation--that is tosay, the operation that permits certain sounds to be attenuated bysuperimposing other sounds that are created in antiphase with the soundsto be attenuated.

The invention finds general application in the soundproofing of a spaceof chosen dimensions, and more particularly the space containing atleast the head of a passenger sitting on a seat of a motorized vehiclesuch as an aircraft or an automobile with a view to improving theacoustic comfort of said passenger.

U.S. Pat. No. 5,133,017 (CAIN) proposes a seat equipped with a personaldevice for active acoustic attenuation which is integrated within thehead-rest of the seat. The device comprises two channels disposed on thefront face of the head-rest of the seat. Each channel possesses aloudspeaker linked to a ring of microphones. Electronic control meansreceive the noise picked up by the two rings of microphones and deliver,after processing, a control signal for noise attenuation for eachloudspeaker in order to create sounds in antiphase with the noise to beattenuated.

The arrangement of the microphones in a ring here permits deteriorationof the results to be avoided in the event of movements of the head ofthe individual or in the case of masking of a particular point of thering by hair or such like. But this avoidance is obtained at the expenseof a complexity of execution (due in particular to the number ofmicrophones to be implemented) and the necessity to calculate the meanvalue of the noises picked up by each ring of microphones in order toimprove the acoustic reproduction of the attenuation control signal.

The present invention provides a remedy for these drawbacks.

The invention focuses on a personal method for active acousticattenuation comprising the following stages:

a) a source is provided that is suitable to deliver an activeattenuation signal in order to reduce an unwanted noise in response to areceived control signal,

b) sensor means are provided for picking up the unwanted noise,

c) an acoustic structure is provided that is suitable to support thesource as well as the sensor means in proximity to the head of anindividual,

d) electronic control means are provided that are suitable to generatethe active attenuation control signal for the source,

e) a selected electrical signal is applied beforehand to the input ofthe source in order to determine at least the impulse response, during apredetermined period, between the output of the source and the input ofthe sensor means, and

f) in real time the active acoustic attenuation control signal isdetermined in accordance with a predetermined processing rule that isestablished at least in accordance with the impulse response asdetermined previously, in order to minimize the energy of the signaldelivered by the sensor means.

According to a general definition of the invention, stage c) consists inconverting said acoustic structure in order to maintain invariant, atleast during the determination of the active attenuation control signaland in the presence of the individual, the volume within which thesensor means and the source are accommodated, permitting the impulseresponse between the output of the source and the input of the sensormeans to be maintained invariant.

In this manner, owing to the invariance of the volume containing thesensor means and the source, and consequently the invariance of theimpulse response between the output of the source and the input of thesensor means, the determination of the active acoustic attenuationsignal has less need for processing.

Another aim of the present invention is a personal device for activeacoustic attenuation in order to implement the method according to theinvention, comprising:

a source that is suitable to deliver an active acoustic attenuationsignal in order to reduce an unwanted noise in response to a receivedcontrol signal,

sensor means for picking up the unwanted noise,

an acoustic structure that is suitable to support the source as well asthe sensor means in proximity to the head of an individual,

electronic control means that are suitable to apply, beforehand, aselected electrical signal to the input of the source in order todetermine at least one impulse response, during a predetermined period,between the output of the source and the input of the sensor means andto determine, in real time, said active attenuation control signal inaccordance with a predetermined processing rule that is established atleast in accordance with the impulse response as determined previously,in order to minimize the energy of the signal delivered by the sensormeans.

According to a general definition of the device according to theinvention, the acoustic structure comprises means of support andprotection in order to maintain invariant, at least during thedetermination of the active acoustic attenuation control signal and inthe presence of the individual, the volume within which the sensor meansand the source are accommodated, permitting the impulse response betweenthe output of the source and the input of the sensor means to bemaintained invariant.

According to a preferred embodiment of the invention the support meanscomprise a casing comprising substantially acoustically opaque walls, anopening intended to be disposed in proximity to the head of theindividual and a partition which is substantially acoustically opaqueand which divides said partition into first and second cavities, thefirst cavity being open in accordance with said opening and containingthe sensor means, whereas the source is supported by the partition insuch a manner that the signal emitted from the source is propagateddirectly into the first cavity, towards the opening in the partition.

In practice, the means of protection comprise a grille or a similardevice which is retractable, covered with a substantially acousticallytransparent material and intended to be folded over the opening in thefirst cavity in order to maintain invariant the volume of said firstcavity, even in the presence of the individual.

According to the invention, the invariance of this volume and thisimpulse response is thus obtained by mechanical means which arerelatively simple and inexpensive and which can be put into effect inproximity to the space to be acoustically attenuated, facilitating theachievement of a sufficient active acoustic attenuation and alsocontributing to reducing the costs of implementing the invention.

The acoustic structure is preferably totally integrated within the upperpart of a seat or a similar object.

The device described in the aforementioned patent U.S. Pat. No.5,133,017 (CAIN) utilizes the technique of active acoustic attenuationby retroaction, also called "feed back", based on the sensing of thesounds emanating from the points disposed in proximity to themicrophones. This technique has a relatively low efficiency, for itattenuates the noises solely around the plurality of the points wherethe sounds are sensed. This technique is therefore not whollysatisfactory for obtaining acoustic attenuation in a space of chosendimensions and, more particularly, the space containing the head of apassenger sitting on a seat.

U.S. Pat. No. 4,977,600 (ZIEGLER) describes another technique for activeacoustic attenuation, called the "synchronous" technique.

The technique here is based on an operation that permits harmonic soundsto be attenuated by causing them to interfere with a predeterminedsignal which is emitted from a synchronous reference and which is inphase opposition.

This signal emitted from a synchronous reference is a signal which issynthetic (for example, generated by a synthesizer driven by thetachometer of the motorization units of the aircraft), global, notsensed, and presumed to correspond to the inverse of the noise to beattenuated. Now, in practice, it is observed that the consistencybetween a measured signal and its modelling (synthetic signal) is goodenough for speech (speech synthesis), but it is very poor for noise.

Such a device can therefore serve only to attenuate a noise composed ofa pure frequency and its harmonics. It does not permit activeattenuation of noise within a broad band of frequencies. This techniquetherefore cannot be used in order to obtain active acoustic attenuationin noisy environments consisting of random noises.

Known furthermore is an active acoustic attenuation based on thetechnique by anticipation also known as "feed forward". Patent FR 8313502 describes such a technique applied in particular to the broadbandnoises propagated along a guide such as a ventilation shaft. In thiscase an active acoustic attenuation is obtained at the shaft output bydetecting upstream the noise emanating from the ventilation motor, byinjecting an opposing noise transversely into the shaft as a function ofthe upstream noise detected in this manner, and by checking theresulting acoustic attenuation by detecting, downstream of the injectionof the opposing noise, the residual noise propagated in the shaft.

The major difference between the "synchronous" technique and thetechnique by anticipation consists in elaborating the opposing noise asa function of a measurement by microphone of the upstream noise beingpropagated in the space to be subject to noise-reduction ("feed forward"technique), instead of a synthetic signal ("synchronous" technique).

It would be theoretically possible to utilize the technique byanticipation for the soundproofing of a space of chosen dimensionscontaining at least the head of a passenger sitting on a seat. But thecomplexity, the interaction of the passenger with the microphones andloudspeakers, the absence of knowledge of the propagation of the noisein this space, and the cost of such a device have meant that thoseskilled in the art have not hitherto contemplated its implementation inthis application.

On the other hand, the applicants have observed that the absence ofknowledge of the propagation of the noise in the space containing thehead of the passenger is not a hindrance, a fact that opens up certainpossibilities.

Thus the method according to the invention is of the type that operatesby anticipation and comprises moreover the following stage:

g) remote sensor means are provided, disposed at a chosen location andsuitable to pick up, in real time, an unwanted noise that is capable ofbeing propagated from said location towards said space, the processingrule being established moreover in accordance with the remote noisepicked up in this manner.

Another object of the invention is a noise-reduced space obtained bymeans of the device according to the invention, said noise-reduced spacebeing independent, local, interchangeable, and of "generous" dimensions,and the active acoustic attenuation of said space being capable of beingaugmented at least partially by that of another noise-reduced spaceoverlapping said space at least partially.

Other characteristics and advantages of the invention will emerge in thelight of the detailed description below and the drawings, in which:

FIG. 1 is a schematic representation of the personal device for activeacoustic attenuation according to the invention,

FIG. 2 represents, in side view and partial section, a seat equippedwith a personal device for attenuation according to the invention,

FIG. 3 is a top view representing schematically the noise-reduced spaceaccording to the invention,

FIG. 4 is a side view, in section, representing schematically thenoise-reduced space according to the invention, and

FIG. 5 is a diagram showing the active acoustic attenuation obtained bymeans of a device according to the invention.

With reference to FIG. 1, the personal device for active acousticattenuation comprises an acoustic structure 2 that is intended toprovide a space 4 for active acoustic attenuation containing at leastthe head (here the ears 6) of an individual 8.

As will be seen in more detail below, the space 4 is advantageously thatwhich contains the head of a passenger sitting on a seat of a motorizedvehicle such as an aircraft or an automobile with a view to improvingthe acoustic comfort of the passenger using said seat. In otherapplications the space for active acoustic attenuation may be thatobtained in the vicinity of a work station or in the vicinity of a wallwithin which the acoustic structure 2 is integrated.

In general the device comprises, in the space 4, a source 30 of soundsand sensor means 20 for picking up sounds. It may be mono-channel ormulti-channel--that is to say, the source 30 may comprise one or moreacoustic transducers such as loudspeakers or similar devices and thesensor means 20 may comprise one or more acoustic transducers such asmicrophones or similar devices (pressure sensors). The device accordingto the invention preferably has two channels, each channel beingassociated with an ear. The references of the constituent elements ofthe channel disposed to the right of the seat are followed by the letterD, whereas the references of the constituent elements of the channeldisposed to the left of the seat are followed by the letter G.

Electronic control means 50 generate the active attenuation controlsignal for the source 30.

In the seat application the electronic control means 50 are fixed at thelevel of the lower part of the seat.

According to the invention, remote sensor means 60 are provided,disposed preferably outside the space 4, in order to pick up an unwantednoise SE which is external to the space and capable of being propagated,preferably without guidance, into the space 4 after a certain delay.These remote sensor means 60 are, for example, fixed to a foot of theseat, as will be seen in more detail below.

It is to be noted that in a variant the device can function with aremote sensor disposed in the space 4 to be subject to noise-reduction.In this case it is advisable to implement an algorithm that filters theperturbation caused by the source on the remote sensor.

In accordance with the basic principle of the active acousticattenuation according to the invention, the control means 50 apply,beforehand, a selected electrical signal to the input of the source inorder to determine at least one impulse response H between the output ofthe source 30 and the input of the sensor means 20. In accordance withsaid impulse response H as determined previously and the remote noise SEas picked up in real time, the electronic control means determine, inreal time, the values of the active attenuation control signal inaccordance with a predetermined rule that is established in order tominimize at least the energy of the signal delivered by the sensormeans.

It is appropriate to note here that the personal device for noiseattenuation according to the invention is distinguished from patent U.S.Pat. No. 4,977,600 cited above by virtue of the fact that the activeattenuation here is broadband in frequency, inasmuch as the signal SEpicked up by the remote sensor means is itself broadband in frequency,unlike the synthetic signal in the narrowband associated with theaforementioned patent. This results in better active acousticattenuation.

In the following description the term `impulse response` in the timedomain is equivalent to the term `transfer function` in the frequencydomain. Likewise, the algorithms described below are those pertaining tothe time domain, but in practice they may pertain to the frequencydomain or to a combination of these two domains.

The detailed structure and the operation of the personal attenuationdevice of the invention, in the case of such a device having twochannels, are the following.

The remote microphone 60 drives the input of a pre-amplifier 22SE. Theoutput of the pre-amplifier 22SE is linked to the input of ananti-overlapping filter 74SE (so-called anti-aliasing filter). Theoutput of the filter 74SE is linked to the input of an analogue/digitalconverter 76SE, the output of which is linked to digital processingmeans 300 via a databus 200.

The analogue/digital converter 76SE operates, for example, with 12 bits.

In the same way, each microphone 20G and 20D drives the input of arespective pre-amplifier 22G and 22D. The output of each pre-amplifier22G and 22D is linked to the input of a respective anti-overlappingfilter 74G and 74D. The output of each filter 74G and 74D is linked tothe input of a respective analogue/digital converter 76G and 76D, theoutput of which is linked to the digital processing means 300 via thedatabus 200.

The anti-overlapping filtering elements 74D, 74G and 74SE are, forexample, filters of the programmable type ELLIPTIQUE of order 7 withswitched capacities.

The output 250 of the digital processing means 300 drives the two activeacoustic attenuation channels, namely the right channel D and the leftchannel G. The right channel D comprises a digital/analogue converter78D, the input of which is linked to the output of the digitalprocessing means 300 and the output of which drives the input of asmoothing filter 79D. The output of the filter 79D is linked to theinput of a power amplifier 90D, the output of which drives theloudspeaker 30D which diffuses the active acoustic attenuation signalSAD. The left channel G is symmetrical to the right channel D andcomprises the same elements as the left channel, namely: adigital/analogue converter 78G, a smoothing filter 79G and a poweramplifier 90G linked to the loudspeaker 30G which diffuses the activeacoustic attenuation signal SAG.

The power amplifiers 90D and 90G have, for example, a nominal power of10 Watts, effective below 8 Ohms. They are advantageously of hi-fiquality. The total ratio of harmonic distortion is, for example, lowerthan 0.2% for an effective power of 10 Watts.

The pre-amplifiers 22D, 22G and 22SE are, for example, standardoperational amplifiers.

The digital processing means 300 comprise, for example, a processor suchas that sold by TEXAS INSTRUMENTS under the reference TMS320P25.

In practice, at the time of installation of the device in the aircraftor otherwise at a manufacturing site it is necessary to effect anadjustment stage, the objective of which is to take up a position at themaximum of the dynamic range of the converters.

This adjustment is carried out, for example, by simulating an unwantednoise to be attenuated which is close to that to be attenuated inoperation, with the aid of an electro-acoustic system (not shown), forexample a system comprising a cassette-reading unit, an amplifier and anacoustic speaker.

The power of the electro-acoustic system is adjusted in order to exhibitin the region of the space 4 a noise level equivalent to that existingin operation, for example when the aircraft is in flight phase.

The electro-acoustic system being in operation, the value of the gain ofthe pre-amplifier 22E is adjusted in order to exhibit at the output ofthis pre-amplifier a signal level close to but substantially lower thanthe signal level admitted by the analogue/digital converter 76SE. Suchan adjustment advantageously permits the maximum of the dynamic range ofthis converter to be attained. This adjustment is repeated for thepre-amplifiers 22D and 22G.

After this adjustment stage, determination of the impulse response iseffected in the following way.

First of all the digital processing means 300 send, successively withrespect to each of the converters 78, a selected identification signal,for example a Dirac signal, a white noise or some other noise.

Advantageously the gain of the right and left amplifiers 90D and 90G isadjusted afterwards for the right and left channels D and G, in orderthat the excitation of the right and left loudspeakers 30D and 30Gproduces at the output of the pre-amplifiers 22S and 22G a signal levelclose to that adjusted in the course of the preceding stage relative tothe dynamic adjustment of the converters.

Once the gains of the two output channels have been adjusted in thismanner, the effects of the loudspeakers on the outputs of themicrophones 20 are determined and the impulse responses betweenloudspeakers and microphones are deduced.

In practice, the impulse responses are measured during a predeterminedperiod, for example on 80 measurement points at the sampling frequencyof 1,000 Hz.

The determination of the impulse responses can be achieved in open-loopconfiguration--that is to say, in a configuration in which theelectro-acoustic system is turned off (that is to say, on the ground).

However, the applicants have observed that the impulse responsesobtained in flight are advantageously substantially equal to those inopen loop, inasmuch as in the seat application, the microphones 20D and20G being close to the loudspeakers 30D and 30G, the direct acousticfield predominates over the reflections.

This is why, according to the invention, the procedure for identifyingthe impulse responses is advantageously executed at a site external tothe application, for example in a production factory.

This results in flexibility of use of the device, inasmuch as it is notnecessary to devise new electronics for each installation of said devicein a structure such as a seat.

Furthermore, the adjustments of the gains are advantageously effected bymeans of potentiometers. In a variant, charts may provide, for a certainnoise to be attenuated, the values of the potentiometers of the inputpre-amplifiers 22D and 22G and of the output pre-amplifiers 90D and 90G.

In the aircraft-seat application, where the noise level to be attenuatedevolves as a function of the flight conditions (acceleration, cruisingspeed, turbulence) and of the aircraft (propellers, jet engines,supersonic, etc), an automatic checking of the gain may adapt,permanently, the values of the input and output gains in order to attainthe maximum of the dynamic range of the input converters 76 and to adaptthe sound level emitted by the loudspeakers.

Under flight conditions the control means 50 acquire, periodically andin real time, the remote noise picked up by the remote sensor means.They also calculate the energy of the signal, which is representative ofthe sum of the energies of the signals delivered by the sensor means 20.

Afterwards, the digital processing means, in particular digitalfiltering elements (not shown), are set in search of the optimalconvergence parameter. The knowledge of the impulse responses measuredpreviously and of the remote noise in real time permits a chosenminimization algorithm to determine, in real time, the values of theactive acoustic attenuation control signal. The aim of the convergencehere is to minimize the energy of the signals delivered by themicrophones that are disposed in the space to be subject tonoise-reduction.

For example, the minimization algorithm utilizes the Least Mean Squarestechnique.

In a configuration with one channel--that is to say, with a singlesensor, for example a microphone 20, and a single source, for example aloudspeaker 30--the determination of the values of the activeattenuation control signal takes into account only the impulse responsebetween the output of the source and the input of the sensor meansmeasured previously and the signal of the remote sensor.

In a configuration with two channels the determination of the values ofthe active acoustic attenuation control signal is substantially similarto that of the configuration with one channel, being distinguished fromthis by the cross-over interactions between the loudspeakers 30G, 30Dand the microphones 20D and 20G. However, these cross-over interactionsare sometimes negligible. Under these conditions only direct impulseresponses are advantageously taken into account--that is to say, theimpulse responses between sensor and source pertaining to the samechannel.

Furthermore it is appropriate to note that it has been considered herethat the remote noise is propagated without alteration as far as thespace to be subject to noise-reduction and that under these conditionsthe noise picked up by the remote sensor means 60 is substantiallyequivalent to that picked up by the sensor means 20.

Now, in practice, changes in the propagation of the remote noise mayoccur.

Under these conditions the determination of the attenuation controlsignal is established in accordance with a rule that takes into accountnot only the impulse response between the source 30 and the sensor means20, the noise picked up by the sensor means 20 and the noise picked upby the remote sensor means 60 (simple case without alteration), but alsothe impulse response between the source 30 and the remote sensor means60 as well as the impulse response between the remote sensor means 60and the sensor means 20.

With reference to FIG. 2, the seat 500 is equipped with the attenuationdevice according to the invention. The seat conventionally comprises abase 504 and a back 506. A head-rest 508 is provided in the upper partof the back 506. This seat 500 is provided for a person.

The skeletal structure of the back 506 is constituted by walls 522 andan armature 524 comprising a plurality of curved tubes disposed betweenthe walls 522. The internal face 526 of the walls 522 is advantageouslycovered with a damping material of mineral-wool type, whereas theexternal face 528 of the walls 522 is advantageously covered with a trimcovering pertaining to the back 506. The tubular armature 524 isaccommodated within a cavity 525 that is recessed on the rear face ofthe walls 522.

The seat is, for example, that of an aircraft passenger or that of anautomobile passenger.

The seat rests on the floor 509 by means of feet 510. It is an entirelyconventional seat. Only its upper part, at the level of the head-rest,is modified in order to receive, in part, the attenuation deviceaccording to the invention.

In the seat application, means 520 support the two loudspeakers 30D and30G and the two microphones 20D and 20G. These means 520 comprise twoidentical casings 520D and 520G, each associated with a channel. Thecasings are semi-closed or closed and are incorporated within thehead-rest 508.

Each casing is called semi-closed because it is delimited by thenon-rigid and open-worked walls 522 of the rear face of the back. Thesewalls are acoustically opaque. They may be made, for example, of glasswool a few centimeters in thickness.

The two semi-closed casings 520D and 520G are separated by a centralpartition 535.

In order to simplify the description, the elements of a single channel(here the left channel) are described. Obviously, this descriptionapplies to the right channel, mutatis mutandis.

Each casing (for example, that associated with the left channel 520G)comprises acoustically opaque walls, an opening towards the outside andin proximity to the head of the individual and an acoustically opaquetransverse partition 531G. This partition 531G divides the casing intotwo half-cavities, namely a front cavity 534G and a rear cavity 532G. Italso supports the loudspeaker 30G. As for the microphone 20G, it isdisposed in the front cavity 534G.

The upper part of the head-rest which forms a sort of cap 527 linkingthe rear cavity 532G to the front cavity 534G is advantageouslyconstituted by a rigid wall in order to halt the propagation of the rearwave of the loudspeaker 30G towards the top of the seat.

An opening 530G is provided in the partition 531G for accommodation ofthe loudspeaker within the casing 520G.

The front cavity 534 is provided in front of the diaphragm 31G of theloudspeaker 30G.

The microphone 20G is accommodated inside the front cavity 534G in orderto be disposed in proximity to the space to be subject tonoise-reduction, in accordance with a geometrical rule which will bedescribed in more detail below.

The applicants have observed that the invariance of the impulse responsebetween the source and the sensor means can be obtained by relativelysimple and inexpensive mechanical means which can be put into effectclose to the space to be acoustically attenuated, facilitating theachievement of a sufficient acoustic attenuation and also contributingto reducing the costs of implementing the invention.

This result is obtained by protection means permitting the semi-closedacoustic casings described above to be protected from acousticinstabilities that arise, for example, from the movement of the head ofthe individual or from mechanical intrusions entering the front cavities534D and 534G from the outside.

With reference to FIG. 2, these protection means comprise a grille 600which is pivotally mounted in relation to a horizontal axis 562 at thelevel of the upper part of the head-rest 508 (the back 506 is consideredas extending along a vertical axis 532). The grille is represented inits open position. In its operational position it is capable of beingfolded over in front of the partitions 531. In this way the grilleprotects the microphones and the diaphragms disposed in each frontcavity 534G and 534D as described above.

It should be noted that the grille delimits the volume of each frontcavity 534, which thus remains invariant, permitting an invariantimpulse response to be obtained. The grille is acoustically transparentand rigid. Its shape conforms advantageously to that of the head-rest.

This grille 600 comprises, for example, rectilinear bars 564 linkingcurvilinear bars 566. In a variant the protection means comprise anopen-worked and acoustically transparent wall.

In practice, provision is made to cover the grille with an open-celled,acoustically transparent foam 602 for the comfort of the passenger, orwith a perforated fire-retardant foam. This foam 602 is advantageouslycovered with a fabric 604 which is likewise acoustically transparent.

Furthermore, the electronic control means 50 that generate theattenuation control signal for the source are accommodated within ahousing 550 that is fixed to the lower face 551 of the base 504.

In practice, the remote sensor means 60 are disposed outside the space 4to be subject to noise-reduction, in order to pick up an undesired noisethat is external to the space 4 and capable of being propagated freelyin said space. These remote sensor means are, for example, a microphone60 fixed to a foot 510 of the seat by means of a support 561.

The remote microphone 60 is advantageously protected from the outside bya grille 563 surrounding said remote microphone.

In a variant the remote microphone 60 may be replaced or complemented byan accelerometer. The information emanating from the accelerometer maybe utilized as a signal that is representative of the noise.

Cables or connections 552 are provided in order to link the controlmeans 50 to the source 30, to the sensors 20, and also to the remotesensor means 60. The device is fully autonomous, inasmuch as itcomprises a power-supply input 554 linked to the on-board power supplyof the aircraft.

In order to obtain a coherence between the sensor means 20 and theexternal sensor means 60, the device according to the invention providesfurthermore a positioning of said means 20 and 60 on supports whichfilter in a passive manner the possible vibrational perturbations thatcome to contaminate the acoustic signals.

In practice, the supports of the microphones are constituted by amaterial of elastomer type.

With reference to FIG. 3, the loudspeakers 30G and 30D are disposed inproximity to the protection grille 600, for example at a distance of afew centimeters.

The geometrical arrangement of the loudspeakers and of the microphonescomplies with a chosen geometrical rule in order to obtain anoise-reduced space of chosen dimensions.

With reference to FIGS. 3 and 4, the applicants have obtainedsignificant results with the following configuration:

distance D1 between the two microphones 20D and 20G, D1=17 cm,

distance D2 between the diaphragm of the loudspeaker 30D and that of theassociated microphone 20G, D2=4 to 5 cm,

distance D3 between the axis of the loudspeaker 30G and the axis of themicrophone 20G=15 cm.

The dimensions of the active acoustic attenuation space vary as afunction of the frequency of the noise to be attenuated (here, forexample, the noise of a turboprop engine of an aircraft).

At frequencies of 68 Hz (associated with the space E1), 136 Hz (E2) and204 Hz (E3), the dimensions of the noise-reduced spaces E1, E2 and E3,exhibiting an attenuation better than or equal to 3 dB, are thefollowing:

length L1 of space E1: 55 cm,

width 11 of spaces E1, E2: 48 cm,

height H1 of spaces E1, E2 and E3: 60 cm,

length L2 of space E2: 67 cm,

length L3 of space E3: 55 cm,

width 13 of space E2 (sic): 23 cm,

width r1 between the microphone 20 and the front end of space E1: 33 cm,and

width r3 between the microphone 20 and the front end of space E3: 23 cm,

height H2 between the microphone 20 and the upper end of space E1 or E2:24 cm.

The following attenuations have been obtained:

attenuation of 12 dB obtained at the fundamental frequency of 68 Hz at adistance of 10 cm from the loudspeaker,

attenuation of 30 dB obtained at the harmonic frequency of 136 Hz at adistance of 10 cm from the loudspeaker,

attenuation of 25 dB obtained at the harmonic frequency of 204 Hz at adistance of 10 cm from the loudspeaker.

Moreover, the applicants have observed that when the distance D2 betweenthe diaphragm of the loudspeaker and that of the associated microphoneincreases, the active acoustic attenuation increases.

With reference to FIG. 5, the noise in dB in the device according to theinvention is represented by a dashed line and the noise with the deviceaccording to the invention is represented by a solid line. These resultswere obtained at a distance of 10 cm from the source, under noisyaverage conditions, in stabilized flight, in a turboprop aircraft.

The applicants have observed that a slight gradient exists in the activeattenuation in the delimited space in comparison with the conventionaltechniques.

Furthermore, the applicants have observed that the noise-reduced spaceobtained in this manner constitutes a space which is noise-reduced,individual, local, interchangeable, and of "generous" dimensions, theactive acoustic attenuation of said space being capable of beingaugmented at least partially by that of another noise-reduced spaceoverlapping said space at least partially and capable of being added toanother noise-reduced space without generating parasitical interference.

Thus by virtue of the invention it is possible to obtain good acousticattenuation in a global volume of chosen dimensions correspondingsubstantially to the union of the individual noise-reduced spaces thusobtained, with the possibility of modifying at will the dimensions ofthe global volume by simple addition or withdrawal of an individualspace and with the possibility of interchanging an individual space withanother in the event of failure of an individual device.

We claim:
 1. A personal method for active acoustic attenuationcomprising the following stages:a) a source (30) is provided that issuitable to deliver an active attenuation signal (SA) in order to reducean unwanted noise in response to a received control signal, b) sensormeans (20) are provided for picking up the unwanted noise, c) anacoustic structure (2) is provided that is suitable to support thesource (30) as well as the sensor means (20) in proximity to the head ofan individual, d) electronic control means (50) are provided that aresuitable to generate the active attenuation control signal for thesource, e) a selected electrical signal is applied beforehand to theinput of the source in order to determine at least the impulse response(H), during a predetermined period, between the output of the source andthe input of the sensor means, and f) in real time the active acousticattenuation control signal is determined in accordance with apredetermined processing rule that is established at least in accordancewith the impulse response as determined previously, in order to minimizethe energy of the signal delivered by the sensor means,wherein stage c)consists in converting said acoustic structure (2) while maintainingmeans of support and protection (520, 600), delimiting a volume (564)where the sensor means (20) and the source (30) are accommodated, saidvolume being maintained invariant, at least during the determination ofthe active attenuation control signal and in the presence of theindividual, permitting the impulse response (H) between the output ofthe source (30) and the input of the sensor means (20) to be maintainedinvariant.
 2. A method as claimed in claim 1, comprising moreover thefollowing stage:g) remote sensor means (60) are provided, disposed at achosen location and suitable to pick up, in real time, an unwanted noisethat is capable of being propagated from said location towards an activeacoustic attenuation space containing at least the head (6) of anindividual (8), the processing rule being established moreover inaccordance with the remote noise picked up in this manner.
 3. A personaldevice for active acoustic attenuation, of the type comprising:a source(30) that is suitable to deliver an active acoustic attenuation signal(SA) in order to reduce an unwanted noise in response to a receivedcontrol signal, sensor means (20) for picking up said unwanted noise, anacoustic structure (2) that is suitable to support the source (30) aswell as the sensor means (20) in proximity to the head of an individual,electronic control means (50) that are suitable to apply, beforehand, aselected electrical signal to the input of the source in order todetermine at least one impulse response (H), during a predeterminedperiod, between the output of the source and the input of the sensormeans and to determine, in real time, said active attenuation controlsignal in accordance with a predetermined processing rule that isestablished at least in accordance with the impulse response asdetermined previously, in order to minimize the energy of the signaldelivered by the sensor means, wherein the acoustic structure (2)comprises means of support and protection (520, 600) delimiting a volume(534) where the sensor means (20) and the source (30) are accommodated,said volume being maintained invariant, at least during thedetermination of the active acoustic attenuation control signal and inthe presence of the individual, permitting the impulse response (H)between the output of the source (30) and the input of the sensor means(20) to be maintained invariant.
 4. A device as claimed in claim 3,wherein the support means (520) comprise a casing comprisingsubstantially acoustically opaque walls, an opening intended to be inproximity to the head of the individual and a partition which issubstantially acoustically opaque and which divides said casing intofirst (534) and second (532) cavities, the first cavity (534) being openin accordance with said opening and containing the sensor means (20),whereas the source is supported by the partition in such a manner thatthe signal emitted from the source is propagated directly into the firstcavity, towards the opening in the casing.
 5. A device as claimed inclaim 4, wherein the means of protection (600) comprise an open-workedgrille or a similar device which is retractable, covered with asubstantially acoustically transparent material and intended to befolded over the opening in the first cavity (534) in order to maintaininvariant the volume of said first cavity (534), even in the presence ofthe individual.
 6. A device as claimed claim 3, wherein the acousticstructure (2) is totally integrated within the upper part of a seat or asimilar object.
 7. A device as claimed in claim 3, wherein the sensormeans (20) comprise at least one microphone or a similar device.
 8. Adevice as claimed in claim 3, comprising moreover remote sensor means(60) disposed at a chosen location and suitable to pick up, in realtime, an unwanted noise that is capable of being propagated from saidlocation towards a space (4) of active acoustic attenuation containingat least the head (6) of an individual (8), and wherein the electroniccontrol means determine the active attenuation control signal inaccordance moreover with the remote noise picked up in this manner.
 9. Adevice as claimed in claim 8, wherein the remote sensor means (60)comprise at least one microphone and/or one accelerometer.
 10. A deviceas claimed in claim 8, wherein the remote sensor means (60) areunidirectional, capable of swivelling and accommodated within astructure (563) that protects against mechanical intrusions.
 11. Adevice as claimed in claim 3, wherein the source (30) comprises at leastone loudspeaker or a similar device.
 12. A device as claimed in claim 3,wherein the sensor means and/or the remote sensor means are mounted ondamping means that are suitable to damp parasitical vibrations.
 13. Anacoustic structure of seat type or the like, comprising a personaldevice for active acoustic attenuation as claimed in claim
 3. 14. Theinstallation of an active acoustic attenuation system comprising aplurality of acoustic structures as claimed in claim 13, disposed inadjacent manner with a view to adding the acoustic attenuations obtainedby said acoustic structures and in this manner obtaining a globalnoise-reduced space resulting from this addition.